2009 Stoddart rotaxanes

2009 Stoddart rotaxanes
05 October 2009 - Supramolecular chemistry

Supramolecular chemist James Fraser Stoddart moved to a new university this year and was dropped from the Thompson shortlist for the Nobel Prize in Chemistry 2009. Lets see what this blog's favorite has been up recently.

Enter the mechanised nanoparticle (MNP): a molecular machine that looks like a spigot capable of controlled release of encapsulated molecules (think drugs). In a recent incarnation the container is a nanoparticle made of mesoporous silica, the controlled substance (or cargo) is Rhodamine B (handy when it comes to spectroscopic identification), the stopper a pseudorotaxane based on viologen (thread) and cucurbituril (movable ring component and capping agent) and the switching trigger is local pH (the bloodstream has a neutral pH, cells are acidic). The cucurbituril unit is water soluble making the whole ensemble all the more biologically relevant (Khashab et al. 2009 DOI). A similar procedure has been described for a propidium iodide cargo with a hexamethylene diamine thread (Angelos et al. 2009 DOI). With two valves, one operating by light and one by pH the MNP can act as a molecular logic gate (Angelos et al. 2009 DOI)

A novel push-button molecular switch was designed with specification single pole, single throw (Spruell et al. 2009 DOI) as part of research into mechanically interlocked molecules. The contraption is based on a catanane (graphic here) where the ring containing a paraquat unit can only clip itself to one tetrathiafulvalene unit in the other ring. Synthesising the catenane required some template-directed synthesis made possible ("threading-followed-by-clipping") by copper assembling two alkyne termini in an Eglington coupling (image here). Reversible clipping and unclipping is a redox reaction. In another system recently developed (Zhao et al. 2009 DOI), one ring in the catenane can now clip itself redox controlled to two molecular units in the other ring (bistable) with both rings tethered to each other by a ethylene glycol unit as in a pretzelane.

Rotaxane complexity is increased when 4 rotaxanes are linked to a single porphyrin molecule and when the stopper contains a disulfide bond as an anchor for nanogold particles (sulfur looooves gold). The tetrameric rotaxane can thus reversibly pick up 4 gold NP's or release them by applying the electrochemical switch in the thread (image here). When the assembly is heated to 95°C, the 4 nanoparticles fuse leaving a tiny hole in the middle where the organic core once was (Olson et al. 2009 DOI). More solid-state chemistry: coat a microcantilever with gold, add a monolayer of redox-controllable rotaxane molecules and an artificial muscle (catilever bending by up to 500 nm) can be demonstrated by applying a voltage (Juluri et al. 2009 DOI). Rotaxanes can also be blended into metal-organic frameworks (Li et al. 2009 DOI)

And that is only a small sample of current Stoddart chemistry. So if ever you are desperate for tetrameric-fused-gold-nanoparticles-with-a-tiny-hole-in-it or a molecule-size-AND-gate, Stoddart is your man.